The present invention relates to a plasma display device used for colored image display of high brightness having low weight and thin construction.
Cathode ray tubes, which have been widely used as image display apparatuses, have bulky construction with large weight and requirements for high supply voltage, and therefore have been replaced by flat panel-shaped image display apparatus such as plasma display device (also referred to as a plasma display panel). The plasma display devices have been developed as the multimedia and telecommunication technologies advance and have been finding new, expanding applications.
The plasma display device is considered to be a promising colored image display apparatus in the future, because it has high image quality achieved by the use of plasma light emission, availability for large screen size and thin and low weight construction without occupying much place to be installed.
A plasma display device, as shown in FIG. 4, has such a construction that a space between a back plate 1, which is a substrate, and a transparent front plate 6 disposed in front of the back plate 1 is divided by plurality of partition walls 2, then to form plurality of light emitting micro-cells 5 surrounded by the partition walls. Each cell includes a pair of discharge electrodes 7 and 7, fluorescent layer 4 applied to the inner wall surfaces within the cell to emit one of the three primary colors, and a rare gas filling the inner space.
An address electrode 3 for switching light emission is placed at the bottom of the cell, and a voltage is applied selectively between the address electrode 3 and the discharge electrode 7, thereby discharging the rare gas to generate plasma. Ultraviolet light emitted by the discharge of the rare gas induces the emission of fluorescent light of wavelengths intrinsic to the fluorescent substances of the fluorescent layer 4 applied to the inner wall of the light emitting cell 5. Such cells constitute as light emitting elements an image for the display apparatus.
The color plasma display device uses emission of light in three primary colors, red (R), green (G) and blue (B) from the different fluorescent substances 4 excited by vacuum ultraviolet rays of the plasma. More particularly, energy released from the rare gas excited by the plasma in the cells, when returning to the ground state, is emitted as vacuum ultraviolet rays, which are used to excite the fluorescent substances 4 and to emit fluorescent light due to a change in energy level of the fluorescent substance from the excited state to the ground state. Red (R), green (G) and blue (B) colors are generated by using light of wavelengths intrinsic to the three different fluorescent substances.
The fluorescent substances which emit different colors receive the supplied energy in the form of the same ultraviolet ray and convert the energies into light of different wavelengths. As a result, the light of different colors have different values of spectral luminous efficacy dependently on the light wavelength, i.e., the color, and, therefore, luminous flux from the light emitting cells varies depending on the color of the cell. Different fluorescent substances also have different luminous efficacy, namely dependency of radiated energy on the electric power supply. Consequently, a simple colored image on the display panel has different value depending on the color, R, G or B.
Supposing, for example, blue light emitted by a fluorescent substance has lower luminance than green light by the another, an image on base of blue, for example color of sea, has different gradient from an image on base of green, for example, the color of forest. The green forest has higher luminance than the blue sea. As a result, gradation of display cannot be controlled smoothly for the image of sea having insufficient luminance, resulting in giving a grained impression of the blue sea to viewers. An image on base of red, for example, making up the color of a person""s skin, has also been difficult to represent with smooth and natural texture for the same reason, because red color has an intermediate level of luminance between green and blue.
Difference in the luminance of fluorescent substances of different colors is a cause of variation in tonality of images displayed on the panel, and the plasma display devices of the prior art has such a problem that it is difficult to represent natural images.
An object of the present invention is to provide a plasma display device which is capable of producing different colors having uniform maximum luminance to display natural full-color images, by setting dimensions of light emitting cells of three colors RGB such that each fluorescent substance in the light emitting cell may emit substantially the same luminous flux of light.
According to the present invention, inner spaces of the light emitting cells of the plasma display device are formed in different sizes according to luminance of the color of each fluorescent substance.
In the present invention, each light emitting cell is to emit light different in color with less deviation in luminous flux between the color light emitting cells, thereby to obtain substantially equal levels of luminance with different colors on the image.
Specifically, among the fluorescent substances of red (R), green (G) and blue (B) colors, cell space for color B, if it has the lowest luminance of the all colors, is made greater, cell space for color G, having the highest level of luminance, is made smaller. Cell space for color R having an intermediate level of luminance is set to an intermediate size. This makes it possible to prevent the image displayed on the plasma display device from being yellowish as in prior art, and to provide more natural full-color display.
More specifically, luminance of light emitted.by a light emitting cell increases in near proportion to the cube of the size of opening of a light emitting cell. For example, when the opening area of the light emitting cell increases by 10%, luminance of light emitted from the cell increases by about 30%. The present invention makes use of this characteristic of light emitting cell to set said cells such that a product of the cube of the size of opening of the light emitting cell emitting one of primary colors multiplied by luminance of the color emitted by the fluorescent substance is substantially equal to that of any other primary color.
According to the present invention, space of the light emitting cell can be changed for different primary colors by forming the light emitting cells with different widths for different primary colors. Width of a light emitting cell can be changed by changing the pitch of partition walls having a constant thickness and/or the thickness of the partition wall having a constant pitch. Such a method may also be employed as the light emitting cells of different primary colors are made to have different depths. In the plasma display device of the present invention, deviation in luminous flux among R, G and B colors emitted by the light emitting cells is mitigated to make the luminances of different colors uniform over the entire display panel, thereby enhancing the displayed image quality.
According to the present invention, ratio of the thickness of each partition wall to the sum of widths of discharging regions located on both sides of the partition wall is preferably made substantially constant. This configuration makes it possible to make substantially equal stress applied to all partition walls regardless of different widths of a light emitting cell adjoining the partition wall (namely the interval between the partition walls). As a consequence, because the stress generated in the partition walls can be made constant even when the opening areas of the light emitting cells and the thickness of the partition walls experience variations because of adjustment of luminance of the three primary colors on the display panel, defects in the partition walls and coupling of the light emitting cells caused thereby can be prevented.